The present invention relates generally to signal processing. In particular, the present invention relates to location determination under the Global Positioning System (GPS) using parameter values obtained from a previous location determination.
Location determination using GPS is achieved in a receiver by processing signals received from multiple GPS satellites. The time difference between signal transmission at a GPS satellite and the same signal being received at the receiver provides a range measurement (xe2x80x9cpseudorangesxe2x80x9d) representative of the distance between the satellite and the receiver. To allow the signal of each GPS satellite to be distinguished from the signals of other GPS satellites, each GPS satellite modulates the carrier signal (1575.42 MHz) with a 1024-chip pseudo-random code (PRN) assigned to that GPS satellite. The PRN has a 1-millisecond period. In theory, the PRNs of the GPS satellites are mutually orthogonal to allow them to be individually recovered at the receiver. Because the position of each satellite at signal transmission time is relatively accurately known, the position of the receiver can be determined in theory using four or more pseudoranges from multiple GPS satellites.
The time difference between signal transmission and the signal reaching the receiver is often represented by the quantity xe2x80x9ccode phase,xe2x80x9d which is equal to the time difference modulo the period of the PRN. To obtain a pseudorange, the receiver processes the signal received, taking into consideration signal and receiver parameters, some of which values are not accurately known a priori. For example, the motion of the satellite relative to the receiver (a signal parameter) and the imperfection in the receiver clock (a receiver parameter) lead to frequency shifts (xe2x80x9cDoppler effectsxe2x80x9d) in the received signal, known respectively as xe2x80x9csatellite Dopplerxe2x80x9d and xe2x80x9cclock Doppler.xe2x80x9d The receiver""s own motion also contributes a Doppler effect in the received signal. Other Doppler effects also exist. For example, a Doppler like effect is introduced when the receiver processes the received signal at an intermediate frequency between the GPS carrier and the base-band frequency. Another Doppler-like effect is introduced by the condition of Earth""s atmosphere. Signal processing is further complicated by the low bit-rate signal which modulates navigational data onto the GPS satellite signal. The navigational data includes the satellite""s ephemeris data, for example, which are used in many systems to aid acquisition of the GPS satellite signal.
In summary, even in ideal conditions, location determination under GPS requires significant signal processing power. Under a less than ideal condition (e.g., weak signal or multipath effects), the signal processing time required to xe2x80x9cacquirexe2x80x9d a location is significant. Thus, methods or techniques that can reduce the signal processing time are desirable.
According to the present invention, a method for providing an initial estimate of an interval representing possible values of a parameter used in acquiring a signal is provided. In that method, the parameter values obtained from previous signal acquisitions and the times at which the signal acquisitions were made are maintained for use in subsequent signal acquisitions. In one embodiment, the maintained parameter values are used to derive a candidate value for that parameter, or another parameter, to be used in a new signal acquisition. To make the maintained values available even after a brief period of power interruption in the receiver, the maintained values can be stored in a non-volatile memory.
In one embodiment, the signal acquisitions correspond to GPS satellite signal acquisitions, and the maintained parameter values are grouped according to whether a successful receiver location determination was accomplished.
The parameters suitable for this method include spatio-temporal parameters, kinematic parameters and other parameters suitable for limiting the search spaces used, or to enhance the level of sensitivity, in a signal acquisition. One example of such a parameter is the clock Doppler, which can be used to limit the carrier frequency search space, or to increase sensitivity, in a GPS satellite signal acquisition. Alternatively, the parameter can be one determined from other parameter values maintained. One example of such a derived parameter is the minimum receiver velocity magnitude.
In one embodiment, the candidate parameter value is then assigned an interval appropriate for the elapsed time since the signal acquisition times associated with the maintained parameter values used to derive the candidate value. Typically, based on the assumption that parameter values change gradually over time, the more recent the maintained values, the tighter the interval can be made. The assigned interval can then be used as an initial estimate to limit the search space or spaces for the next signal acquisition, or to focus the beginning of the search. That is, the information is applicable not only for limiting the search boundary, it can also be used to determine the initial conditions of the search. The suitable interval to be assigned can be determined from empirical data, and known physical or other constraints can be used to limit its range.
Of course, the use of an interval is merely one useful format for expressing a range of parameter values. Other means for expressing parameter values or applications of such parameter values exist. For example, rather than providing an interval or a set of parameter values, one or more constraints on one or more parameters can be output as a result of analyzing parameter values from previous signal acquisitions.
Using data from previous signal acquisitions, the search space associated with the signal processing can be substantially limited, thereby significantly speeding up or increasing the sensitivity, of the signal acquisition process.
The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.